Unilateral renal agenesis (URA) is a common developmental defect in humans, occurring at a frequency of approximately 1 in 500-1000 live births. The frequency of URA in offspring of affected individuals is markedly higher than in the general population, indicating that genetic factors contribute to the genesis of URA. ACI rats spontaneously exhibit URA at an incidence of approximately 15%. In a series of genetic crosses between the ACI strain and two unaffected rat strains, Copenhagen (COP) and Brown Norway (BN), we have mapped to rat chromosome 14 (RNO14) a genetic locus, designated Renag1 (Renal agenesis 1), mat serves as the major, if not sole, determinant of URA in these crosses. The ACI allele of Renag 1 acts in an incompletely dominant and incompletely penetrant manner to confer URA. The overall goals of this proposed research are to define fully the genetic and embryologic bases of renal agenesis in the ACI rat. Specific Aim 1 is to fine map the location of Renagl on rat chromosome 14. We have mapped Renag1 to an approximate 15 centiMorgan (cM) interval on RNO14. To define the location of Renag1 to a resolution approaching 0.5 cM, we will generate up to 400 URA-affected (BN x ACI)F2 progeny and determine the genotypes of these rats at markers placed at a high density across the Renag1 region. Homozygosity for the BN allele at a marker will exclude that marker from residing within Renag1. Specific Aim 2 is to define the embryologic bases of renal agenesis in the ACI rat ACI females will be sacrificed at defined time points after mating and the developing embryos will be harvested, sectioned and examined microscopically. Knowledge of when the first discernable defect in renal development occurs in the ACI rat will provide novel and potentially important insights into when and where the molecular defect associated with Renag1 is manifested. Specific Aim 3 is to compare expression of Pax2, Gdnf and Ret during renal development in the ACI and BN rat strains. Proper renal development requires coordinated expression of Pax2, Gdnf and Ret. We hypothesize that Renag1-associated defect leading to URA in the ACI rat may result in qualitative and/or quantitative differences in Pax2, Gdnf and/or Ret expression relative to the BN rat. Expression of these genes during renal development will be evaluated at the protein and mRNA levels in both the ACI and BN rat strains. Upon completion of these Aims, we will better understand the genetic bases of renal agenesis in the ACI rat and will be able to assess the relevance of this animal model to renal agenesis in humans.